In an example of a conventional image forming apparatus, an electrostatic capacitance detection apparatus detects a remaining amount of toner in a toner container. For example, in an apparatus for detecting a remaining amount of toner described in PTL 1, a flexible member is fixedly connected to one end of an agitation member for agitating a toner in a toner container, a member to be detected is secured to a tip of the flexible member, and an electrostatic capacitance detection apparatus is placed in a lower part of the toner container. The flexible member connected to the agitation member is rotated with rotation of the agitation member and enters the toner. If a toner surface in the toner container is higher than a connecting position between the flexible member and the agitation member, the flexible member enters the toner at a connecting portion to the agitation member, the entire flexible member is flexibly deformed, and rotated along the same trajectory as the connecting portion in the toner. Thus, the member to be detected at the tip of the flexible member is rotated along the same trajectory as the flexible member. However, if an amount of toner is reduced, and the toner surface is lower than the connecting position between the flexible member and the agitation member, and the connecting portion of the agitation member does not enter the toner, the tip and therearound of the flexible member slides on the toner surface, and the member to be detected also slides on the toner surface. With decreasing remaining amount of toner, a height of the toner surface in the toner container is gradually reduced, and a position of the member to be detected sliding on the toner surface is also lowered. Specifically, when the amount of toner is reduced to less than a certain amount, the position of the member to be detected moving on the toner surface is lowered according to the remaining amount of toner, and brought close to a bottom surface of the toner container.
Meanwhile, the electrostatic capacitance detection apparatus can detect electrostatic capacitance between the electrostatic capacitance detection apparatus and the member to be detected moving on the toner surface. The electrostatic capacitance between the electrostatic capacitance detection apparatus and the member to be detected changes depending on a distance therebetween. The electrostatic capacitance detection apparatus is placed in the lower part of the toner container, and with decreasing amount of toner, the height of the toner surface is gradually reduced, and the position of the member to be detected on the toner surface is also lowered. This reduces the distance between the electrostatic capacitance detection apparatus and the member to be detected to reduce the electrostatic capacitance therebetween. Specifically, the electrostatic capacitance between the electrostatic capacitance detection apparatus and the member to be detected changes depending on the remaining amount of toner.
An example of an apparatus for detecting an amount of toner in a developing unit uses a magnetic permeability sensor. For example, PTL 1 is an example of an apparatus for detecting an amount of toner using a magnetic permeability sensor. PTL 2 discloses a toner amount detection apparatus using a first agitation blade that is flexible and may be deformed rearward in a rotational direction by agitation of toner, a second agitation blade that is rigid and placed on a rear side in the rotational direction of the first agitation blade, and a magnetic permeability sensor placed on an outer side of a bottom of a developing unit. The apparatus detects a state of rotation of a metal material placed on each agitation blade using the magnetic permeability sensor placed on the outer side of the bottom of the developing unit. This apparatus is configured so that in the case with a large amount of toner in the developing unit, the first agitation blade and the second agitation blade are integrally rotated, and in the case with a small amount of toner in the developing unit, the first agitation blade and the second agitation blade are separately rotated without being deformed. At this time, the magnetic permeability sensor is used to detect once a change in the magnetic permeability for one turn of the rotation axis in the case with a large amount of toner in the developing unit, and twice in the case with a small amount of toner in the developing unit. The toner amount detection apparatus detects the amount of toner in the developing unit based on the change in the number of times of detection.